Dry grinding of ceramics



United States Patent 3,252,810 DRY GRINDING OF CERAMICS Arthur V. gamers, Flushing, Mich assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing. .Filed Jan. 23, 1963, Ser. No. 253,257 16 Claims. (Cl. 106 -39) This invention relates to ceramics and to an improved method for their manufacture. The invention has particular utility for the manufacture of industrial ceramics, electrical insulators and the like such as are made of high alumina compositions but is also useful for the manufacture of china, tile and other porcelain or high clay content ceramic bodies.

At the present state of the art there are principally three methods used for the manufacture of ceramic articles. The oldest of these methods includes the steps of wet grinding a ceramic raw batch (i.e., grinding with a substantial amount of Water present such that the batch is in the form of a slurry), filtering, aging the filter cake until it has the right plasticity, forming into desired shapes and then drying and firing. This method which requires that the batch contain clay or a like plastic ingredient is used widely for the manufacture of various types of white ware, tile and the like and is also still used to some extent to manufacture spark plug insulators and other high alumina industrial ceramics. However, for the latter types of products, the more widely used method during recent years has been the so-called spray drying and isostatic molding process. With this process the raw batch is first either wet ground or is dry ground (i.e., grinding substantially dry or with not more than about 1% uncombined water present or with not more than about 6% organic material such that the batch is in a loose pulverant state) after which it is mixed with a wax-water emulsion in a blunging operation to form a slurry. Next the slurry is screened and then sprayed in small droplet form into a hot air drying tower to evaporate the water and provide a flowable mass of small globules of waxbonded ceramic. This material is then isostatically molded in rubber molds by the application of fluid pressure to form the desired shapes after which the shaped pieces are fired to burn out the wax and sinter or vitrify the ceramic. This method, which is disclosed in United States Patent 2,251,454, has the advantage that is does not rely on plasticity in the ceramic body composition for purposes of shaping the articles. Hence it can be used to produce high alumina or other pure oxide or mixed oxide bodies which contain little or no clay. The third commonly used method involves the steps of grinding the raw ceramic batch, uniformly admixing with the ground batch a thermoplastic organic resin, injection molding the articles and then firing to burn out the resin and sinter the ceramic. The success of this process is largely dependent on attaining a uniform mixture of the resin with the ceramic and in attaining a uniform density in the molded prefired article.

It will be noted that all the above-outlined methods consist of at least four steps, at least one of Which steps has no other function than to enable the forming of the ground ceramic into self-sustaining prefired articles of relatively uniform density. Uniform density is important because if there should be nonuniformity by reason of air inclusions, nonuniform compaction or the like, there is considerable distortion of the ware during firing as well as inferior properties of mechanical strength and electrical resistance in the finished product. Needless to say, the number and nature of the various steps used in these existing manufacturing methods add considerably to the cost of the finished product.

3,252,810 Patented May 24, 1966 ice The present invention has as its principal object the amount of a long chain organic fatty acid compound which serves as a deflocculating agent during grinding thereby inhibiting agglomeration of the grains and providing the batch with fiowability comparable to that of a liquid. An organic binder may additionally be admixed into the raw batch during the grinding operation to add green strength to the articles subsequently pressed from the batch. I have found that there results from such grinding operation a ceramic batch which can, without any intermediate operations, be pressed or isostatically molded to prefired shapes of extremely uniform density. There remains then only the firing step to sinter or vitrify the shaped articles to the ceramic end product.

The above and other features and advantages of the invention will appear more clearly from the following detailed discussion of particulars of preferred embodiments.

CERAMIC RAW MATERIAL FOR THE GRINDING OPERATION The ceramic material selected for the raw batch will, of course, depend upon the type of ceramic Ware desired. For high strength industrial ceramics or electrical insulators it can, for example, be a single oxide such as aluminum oxide or beryllium oxide, a mixture of such oxides, for example aluminum oxide and magnesium oxide, or a mixture of such oxides together with small amounts of fluxing ingredients of mineralizers such as talc, calcium carbonate, clay and the like. For such items as tile and dinnerware the ceramic raw batch can likewise be of any of the various compositions presently used.

The method .of this invention has the special advantage of being able to produce an extremely high quality ceramic, especially for industrial and electrical uses, where the raw batch is substantially free of all glass-forming ingredients such as silica, as such or in any of its combined forms. Typical examples of such a batchare: aluminum oxide; 100% beryllium oxide; 99.75% aluminum oxide and 0.25% magnesium oxide (added as such or as magnesium sulfate or magnesium nitrate); 99.25% aluminum oxide, 0.25% magnesium oxide (added as such or as magnesium sulfate or magnesium nitrate) and 0.5% chromium oxide.

Where aluminum oxide is the principal ingredient in the raw batch as is desirable where the method is used to produce industrial ceramics or spark plug or other electrical insulators, it is preferred that the aluminum oxide be a substantially soda-free and silica-free calcined alumina of small crystal size. Such aluminas are common in the market.

NONCERAMIC RAW BATCH INGREDIENTS The raw batch for the grinding operation also inclucles a small amount, up to about 6% by weight, of a long chain (i.e., more than six carbon atoms) organic fatty acid salt or ester which serves as a defiocculating agent, i.e., it serves as a nonagglomerator and lubricant and hence inhibits any sticking of the grains one to the other during grinding thereby preventing any packing in the grinding mill and providing the batch with a flowability comparable to that of a liquid both during the grinding operation and during the subsequent pressing operation. The following are examples of such deflocculating agents: the fatty acid or hydrogenated fatty acid esters of glycerol, the metal salts of stearic acid or of stearic acid with a substituted hydroxy group, diglycol monolaurate, diglycol dilaurate. I have found especially desirable for the practice of the invention hydrogenated cottonseed oil, hydrogenated castor oil, aluminum stearate, aluminum hydroxy stearate, a mixture of diglycol monolaurate, diglycol dilaurate and potassium laurate, and polyethylene glycol monolaurate wherein the polyethylene glycol has a molecular weight of about 400.

It is sometimes desirable that the raw batch additionally include a small amount up to about 6% by weight of an organic binder such that the prefired articles resulting from the pressing operation have added mechanical strength to withstand a considerable amount of shock during handling operations between forming and firing. Where a binder is used, it can be mixed into the raw batch prior to or during the grinding operation. Dry powdered paraffin wax is eminently suitable as a binder material.

To attain minimum shrinkage during firing and also for reasons of economy it is, of course, desirable to use as little organic material in thebatch as is necessary to accomplish the desired batch properties. Hence, whereas larger amounts of deflocculating agent and binder can be included in the batch, it is preferred that the total percentage of organic material, including both deflocculating agent and binder, not exceed about 6%. Generally about 2% by Weight deflocculating agent is ample to provide the anti-agglomerating and fiowability properties required and about 2% by weight binder is sufficient for excellent green strength. Where the hydrogenated vegetable oils are used as deflocculating agents, about 2% is adequate not only to prevent agglomeration and packing but also to provide sufficient green strength without addition of wax or other binder.

THE GRINDING OPERATION The grinding operation can be performed in a conventional ball mill preferably either rubber lined or lined with a ceramic material having a composition similar to that being manufactured in order to avoid adverse contamination. In this same vein, it is preferred that the mill balls be of a dense ceramic having a composition the same or similar to the ceramic being manufactured. Mill balls, having a initial diameter of about one to four inches, are quite satisfactory for milling production size batches. There is, of course, mill ball wear during the grinding though I have found that the amount of wear is considerably less in the practice of the present invention than in conventional wet milling operations.

The ceramic ingredients in the desired proportions along with the deflocculating agent and preferably also an organic binder are placed in the mill along with the conventional number of mill balls (approximately four or five pounds of mill balls for every pound of ceramic batch) and the batch is then dry ground until mixing is completed and the desired extent of particle size reduction is accomplished. A grinding time of four to five hours is generally sufficient for compositions containing up to about 95% alumina together with siliceous or other glass forming materials. For higher alumina bodies containing little or no glass forming constituents, grinding times of 10 to 30 hours are desirable. After grinding, the loose, flowable, ground batch can be immediately passed to the pressing or molding operation to produce shaped, self-sustaining prefired articles. The deflocculating agent along with the binder, if one is used, appears in the ground batch as an extremely-thin coating on the ceramic particles.

While a conventional mill ball-to-batch weight ratio of 4 or 5 to 1 is satisfactory to provide good quality ceramic ware, an outstanding product improvement can FORMING OPERATION The desired prefired articles can be pressed directly from the particulate material from the grinding operation preferably by pressing in matched metal dies or by isostatically molding with rubber molds substantially the same as those used in the so-called spray drying-isostatic molding process as previously outlined. Because of the fluid nature of the particulate material from the grinding operation, it fills the die or mold easily, much as would a liquid, and on the application of pressure extremely uniform density can be accomplished in the prefired article. Pressures on the order of 4000 to 7000 pounds per square inch are quite satisfactory to produce the desired results. Even though the ceramic batch material prior to pressing has a high degree of fluidity due to the presence of the deflocculating agent, the prefired articles resulting from the forming operation are self-sustaining and have ample green strength to withstand all the normal handling operations between forming and firing. As indicated previously a binder, preferably powdered dry paraffin wax, should be included in the batch if the deflocculating agent does not provide the amount of green trength desired in the prefired articles.

For the manufacture of ceramic articles of relatively simple shape such as cylinders or discs, the use of matched steel dies serves eminently well and is recommended for the forming operation. However, where the shapes desired are relatively complex, for example, spark plug insulators, the use of rubber molds is much preferred.

In addition to providing an extremely uniform density in the prefired articles, the process of this invention is advantageous in that extremely good compaction can be attained. This is presumably because of the lubricity of the material which allows one grain to slide with respect to the others during the application of pressure thereby resulting in prefired articles of relatively high density. Also, there is evidence that the presence of the deflocculating agent during milling effects the particle size and particle size distribution attained such that the ground material is susceptible to high compaction values. As indicated previously, exceptionally high prefired densities can be accomplished by utilizing the grinding techniques as covered by my copending United States Serial Number 253,255 wherein a large ratio of mill balls-tocharge is used in the grinding operation.

FIRING OPERATION The firing temperature and schedule used to fire the articles from the forming operation to cause sintering will, of course, depend on the ceramic ingredients used. For any type ceramic body made by the process of this invention the firing temperature, time and other conditions can be the same as those utilized in firing the same type of ceramic processed by conventional techniques. For example, for high alumina bodies containing upwards of aluminum oxide, firing temperatures of about 2700 F. to 3100 F. on a six-hour schedule are usual, all as will be evident from the following examples:

Example I A ceramic raw batch was formed by adding to a conventional ball mill calcined aluminum oxide 99.25% (by weight), magnesium oxide 0.25% and chromium oxide 0.5%. Then hydrogenated castor oil in an amount equal to about 2% of the weight of the ceramic batch was additionally added to the mill. The mixture was dry milled for hours using alumina grinding balls in a ratio of about 30 pounds of balls to each pound of batch. I

Example II Same as Example I except that: the ceramic batch consisted of about 90% (by weight) fused alumina,4.5% clay, 4% talc and 1.5% strontium carbonate to which was added 1.5 (of the weight of the batch) aluminum stearate and 2% dry parafiin wax; the batch was ground for 5 hours using a grinding ball-to-batch weight ratio of 4 to 1; the formed cylinders were fired to a temperature of 2950 F.

While the process of this invention involves only three essential steps in proceeding from the initial raw ceramic materials to the fired articles, it will be understood that additional steps may be included if desired. For example, subsequent to the pressing operation and prior to firing it may be desirable to use a form grinding or shaping step to remove portions of the pressed articles particularly where the bodies to be manufactured are of complicated shape. Hence, while the invention has been described in detail with reference to certain embodiments thereof, changes may be made, all within the full and intended scope of the claims which follow.

I claim:

1. In a process for manufacturing ceramic articles including the steps or grinding a particulate ceramic batch, forming a compacted article and firing said compacted article, the step of dry grinding in a ball mill a particulate ceramic batch in admixture with a small but effective amount of an organic material having deflocculant and binder properties, said material having sutficient deflocculating abilityto prevent packing in the ball milland ,sufiicient binding ability to afford green strength for a compacted ceramic article, said material containing a quantity of an organic compound selected from the group consisting of the esters and salts of long chain organic fatty acids having sufiicient deflocculating ability to prevent packing in the ball mill, to produce a free flowing, finely divided ceramic powder.

2. A process as set forth in claim 1 wherein the amount of material ranges from a small but effective amount up to 6.0 weight percent.

3. A process according to claim 1 in which said material consisting essentially of a mixture of parafiin wax and a quantity of an organic compound selected from the group consisting of the esters and salts of long chain fatty acids having sufiicient deflocculating ability to prevent packing in the ball mill.

4. In a process for manufacturing ceramics, the steps of dry grinding in a ball mill a particulate ceramic batch in admixture with a small but effective amount of an organic material having deflocculant and binder properties, said material having suflicient deflocculating ability to prevent packing in a ball mill and sufficient binding ability to afford green strength for a compacted ceramic article, said material containing a quantity of an organic compound selected from the group consisting of the esters and salts of long chain organic fatty acids having sufficient deflocculating ability to prevent packing in the ball mill, to produce a free flowing, finely divided powder, subsequently pressing the ceramic particles from said grinding step to form a compacted article of uniform density, and then firing said article to form a dense, nonporous, sintered ceramic body.

5. A process as set forth in claim 4 wherein the amount of material is from a small but effective amount up to 6.0 weight percent.

6. A process as set forth in claim 4 in which said material consisting essentially of a mixture of paraffin wax and a quantity of an organic compound selected from the group consisting of the esters and salts of the long chain fatty acids having sufiicient deflocculating ability to prevent packing in the ball mill.

7. In a process for manufacturing ceramics, the steps of dry grinding in a ball mill a particulate ceramic batch in admixture with a small but effective amount of an organic material having deflocculant and binder properties, said material having sufficient deflocculating ability to prevent packing in a ball mill and sufiicient binding ability to afford green strength for a compacted ceramic article, said material containing a quantity of an organic compound selected from the group consisting of the metal salts of long chain organic fatty acids having sufficient deflocculating ability to prevent packing in the ball mill, to produce a free flowing, finely divided powder, subsequently pressing the ceramic particles from said grinding step to form a compacted article of uniform density, and then firing said articles to form a dense, nonporous, sintered ceramic body.

8. A process according to claim 7 in which said material consisting essentially of a mixture of parafiin wax and a quantity of an organic compound selected from the group consisting of the metal salts of the long chain fatty acids having sufficient deflocculating ability to prevent packing in the ball mill.

9. In a process for manufacturing ceramics, the steps of dry grinding in a ball mill a particulate ceramic batch in admixture with a small but effective amount of an organic material having deflocculant and binder properties, said material having sufficient deflocculating ability to prevent packing in a ball mill and sufficient binding ability to afford green strength for a compacted ceramic article, said material containing a quantity of aluminum stearate having sutficient deflocculating ability to prevent packing in the ball mill, to produce a free flowing, finely divided powder, subsequently pressing the ceramic particles from said grinding step to form a compacted article of uniform density, and then firing said articles to form a dense, nonporous, sintered ceramic body.

10. A process according to claim 9 in which said material consisting essentially of a mixture of parafiin wax and a quantity of aluminum stearate having sufficient deflocculating ability to prevent packing in the ball mill.

11. In a process for manufacturing ceramics, the steps of dry grinding in a ball mill a particulate ceramic batch in admixture with a small but effective amount of an organic material having deflocculant and binder properties, said material having sufficient deflocculating ability to prevent packing in a ball mill and sufiicient binding ability to afford green strength for a compacted ceramic article, said material containing a quantity of compound selected from the group consisting of glycol and glycerol esters of long chain organic fatty acids having sufiicient deflocculating ability to prevent packing in the ball mill, to produce a free flowing, finely divided powder, subse quently pressing the ceramic particles from said grinding step to form a compacted article of uniform density, and then firing said articles to form a dense, nonporous, sintered ceramic body.

12. In a process according to claim 11 in which said material is diglycol monolaurate.

13. In a process for manufacturing ceramics, the steps of dry grinding in a ball mill a particulate ceramic batch in admixture with a small but effective amount of an organic material having defiocculant and binder properties, said material having sufficient deflocculating ability to prevent packing in a ball and sufficient binding ability to afford green strength for a compacted ceramic article,

said material containing a quantity of hydrogenated cottonseed oil having sufficient defiocculating ability to prevent packing in the ball mill, to produce a free flowing, finely divided powder, subsequently pressing the ceramic particles from said grinding step to form a compacted article of uniform density, and then firing said articles to form a dense, nonporous, sintered ceramic body.

14. In a process for manufacturing ceramics, the steps of dry grinding in a ball mill a particulate ceramic batch in admixture with a small but effective amount of an organic material having deflocculant and binder properties, said material having suflicientdeflocculatin-g ability to prevent packing in a ball mill and sufiicient binding ability to afford green strength for pressing a ceramic article, said material containing a quantity of hydrogenated castor oil having sufiicient defiocculating ability to prevent packing in the ball mill, to produce a free flowing, finely divided powder, subsequently pressing the ceramic particles from said grinding step to form a compacted article of uniform density, and then firing said articles to form a dense, nonporous, sintered ceramic body.

15. Ceramic particles useful for the manufacture of dense, nonporous fired ceramic bodies, said ceramic particles consisting predominantly of alumina particles coated with a material having deflocculant and binder properties, said material providing sufiicient deflocculating ability to prevent packing in a ball mill and sutficient binding ability to afford green strength for a compacted ceramic article, said material consisting substantially of hydrogenated cottonseed oil, said ceramic particles being dry milledin the presence of said material.

16. Ceramic particles useful for the manufacture of dense, nonporous fired ceramic bodies, said ceramic particles consisting predominantly of alumina particles coated with a material having a deflocculant and binder properties, said material providing sufiicient deflocculating ability to prevent packing in a ball mill and sufficient binding ability to afford green strength for a compacted ceramic article, said material consisting substantially of hydrogenated castor oil, said ceramic particles being dry milled in the presence of said material.

References Cited by the Examiner UNITED STATES PATENTS 1,847,196 3/1932 Scott 26463 1,946,052 2/1934 Baldwin 106308 2,113,539 4/1938 Meister 106308 2,225,147 12/1940 Bechtold 24116 2,251,454 8/1941 Jeffery 26456 2,382,136 8/1945 Crowley et al. 264-63 2,386,885 10/1945 Downs et al 106308 2,668,749 2/ 1954 McHan 196308 2,939,199 6/1960 Strivens 26463 2,947,056 8/1960 Csordas et al. 106-65 2,948,632 9/1960 Albert et al. 10672 X 2,968,551 1/1961 North et al. 26463 3,026,210 3/1962 Coble 106-65 3,051,566 8/1962 Schwarz 264--63 FOREIGN PATENTS 406,513 3/1934 Great Britain. 432,032 7/ 1935 Great Britain.

TOBIAS E. LEVOW, Primary Examiner.

H. M. MCCARTHY, Assistant Examiner. 

1. IN A PROCESS FOR MANUFACTURING CERAMIC ARTICLES INCLUDING THE STEPS OR GRINDING A PARTICULATE CERAMIC BATCH, FORMING A COMPACTED ARTICLE AND FIRING SAID COMPACTED ARTICLE, THE STEP OF DRY GRINDING IN A BALL MILL A PARTICULATE CERMIC BATCH IN ADMIXTURE WITH A SMALL BUT EFFECTIVE AMOUNT OF AN ORGANIC MATERIAL HAVING DEFLOCCULANT AND BINDER PROPERTIES, SAID MATERIAL HAVING SUFFICIENT DEFLOCCULATING ABILITY TO PREVENT PACKING IN THE BALL MILL AND SUFFICIENT BINDING ABILITY TO AFFORD GREEN STRENGTH FOR A COMPACTED CERAMIC ARTICLE, SAID MATERIAL CONTAINING A QUANTITY OF AN ORGANIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF THE ESTERS AND SALTS OF LONG CHAIN ORGANIC FATTY ACIDS HAVING SUFFICIENT DEFLOCCULATING ABILITY TO PREVENT PACKING IN THE BALL MILL, TO PRODUCE A FREE FLOWING, FINELY DIVIDED CERAMIC POWDER. 